Investments (Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale, Amortized Cost
Due in one year or less	$ 0
Due after one year through five years	76,027
Due after five years through ten years	207,757
Due after ten years	630,194
Total	913,978
Available-for-sale, Fair Value
Due in one year or less	0
Due after one year through five years	76,341
Due after five years through ten years	208,866
Due after ten years	632,911
Total	918,118
Due in one year or less	0
Held-to-maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	512
Due after five years through ten years	790
Due after ten years	0
HTM Amortized Cost	1,302	$ 1,307
Held-to-maturity, Fair Value
Debt Securities, Held-to-maturity, Maturity, Remainder of Fiscal Year, Fair Value	0
Due after one year through five years	532
Due after five years through ten years	827
Due after ten years	0
Debt Securities, Held-to-maturity, Fair Value	$ 1,359	$ 1,291